Conventionally, an injection molding machine has been known, which has a detector for detecting resin pressure, i.e., injection pressure, hold pressure and back pressure, and performs pressure control in accordance with the detected resin pressure so as to produce good molding products. In general, a pressure sensor disposed within a mold cavity has been employed for detection of the resin pressure. However, this type of sensor is high-priced. In this respect, a strain detector, disposed on a housing which axially supports a screw shaft, for receiving the reaction force from the resin material to detect the resin pressure, has been proposed.
As shown in FIG. 5, this type of detector comprises an annular member 1 and a detecting element such as a strain gauge (not shown) affixed thereon. The annular member 1 is disposed between a thrust ball bearing 3 and a stepped portion 5 formed in a metering bearing housing 2 which is axially and radially supported by the bearing 3 and an angular ball bearing 4, respectively. In assembling the above-mentioned arrangement in which the annular member 1 is interposed between the bearing 3 and the stepped portion 5 within the housing 2, lead wires of the strain gauge must be passed through the housing 2, and an axial clamping force is inevitably applied to the annular member 1 upon assemblage thereof and even thereafter. As a consequence, the annular member 1 is deformed at a portion on which the strain gauge is affixed. Thus, an output from the strain gauge is produced even before the resin pressure is actually generated, that is, a shift of zero point in resin pressure detection takes place. In this connection, a required zero adjustment is made by means of an appropriate circuit in the resin pressure control system upon detection of the resin pressure. Moreover, since the annular member 1 is disposed within a hollowed interior of the metering bearing housing 2, it is troublesome or difficult to effect replacement and maintenance of the annular member 1 and wiring of the lead wires of the strain gauge.